Through a long development from old time the common types of hydraulic turbines have reached a stage of development where most practical designs follow rather established conventions and rules, whereby only small improvements have been obtained during the recent years. In this field however great gains are possible even with relatively small improvements, for example of the efficiency.
The present invention is directed to runners for Francis-type turbines, where in particular the blade shape can have a quite complicated geometry with many parameters, in the first place in association with the runner hub and the surrounding band or ring to which the blades are attached. In addition to efficiency there are in particular cavitation problems that challenge people skilled in the art in attempts to improve the properties of Francis-type runners. As a consequence of the complicated geometrical relationships, water flow patterns and many parameters involved, it is difficult to predict how changes in blade geometry will affect the desired properties. Besides it is expensive and time consuming to carry out experiments with varying geometry and parameters, even if scaled-down models are used.
Thus, more specifically the invention relates to a runner for a Francis-type hydraulic turbine, comprising a ring or band, a hub and a number of blades having a curved shape and attached to the ring and the hub, where each blade has an inlet edge adapted to face towards an upstream guide apparatus in the turbine and an outlet edge adapted to face towards a downstream outlet opening.
Runners in conventional Francis-type turbines have a typical shape and orientation of the blades, characterized by a positive blade leaning at their inlet edge and an outlet edge extending in radial direction. Traditional Francis runners have the weakness that at the suction side of the blade along the ring there is generated a low pressure zone that can lead to cavitation and unfavourable flow conditions introducing losses (reduction of the hydraulic efficiency of the turbine). The cause of this is, inter alia, cross flow at the pressure side, namely that the water does not follow the theoretically favourable flow paths, but takes a more axial direction at the pressure side of the blade. This cross flow is mainly due to a larger pressure difference between surface portions adjacent to the hub and the ring respectively, at the pressure side of the blade. It has been attempted to solve the cavitation problem for example by placing the attachment of the inlet edge at the ring locally more forward in the rotational direction of the runner. Such a solution has been described in particular in Norwegian patent No. 163,378 (corresponding to U.S. Pat. No. 4,479,757). The design according to the patent reduces the problems of cavitation at the blade inlet towards the ring, but will to a small degree reduce the unfavourable cross flow along the pressure side of the blades.
Whereas the patent specification referred to above has been directed to the inlet edge of the blades, the present applicants in earlier runner designs for Francis turbines have employed a specific design with respect to the outlet edge of the blades, namely with such an outlet edge having an inclination deviating from the radial direction, i.e. with an angle of inclination forwardly in the rotational direction of the runner as seen from the hub towards the ring. This has a stabilizing effect on the water flow through the turbine.